One of the most common types of residential retaining walls found in Seattle is the simple concrete slab. Concrete has been used as a building material for literally centuries, and is a natural choice for this application. Perhaps the two most important characteristics of concrete is that it is strong in compression, but weak in tension. The weakness in tension is the reason that reinforcement bars or "rebar" is often added to concrete. I suspect, but cannot prove, that most residential retaining walls in Seattle don't include rebar.
First off, it's often fairly easy to judge the age of a retaining wall just by looking at it. Older walls are often quite dark, and have a coarser texture. They are often covered in moss. Perhaps the easiest way of all to judge the age of a retaining wall is by the property that it's attached to! How old is the house or building that it bounds? This can sometimes be deceptive, as houses and buildings can be remodeled and so look newer on the outside than they really are.
The fundamental purpose of ANY retaining wall is to hold back earth. This sounds like a dead simple task until you begin to think about the physics involved. Earth, or at least the kind we have here in Seattle, is not rock solid. It has fluid characterists, and this effect is intensified by the large amount of rainfall we get here. When it rains heavily in Seattle two things happen; the mass of the earth that the wall is called upon to retain increases by the weight of the added water, and the earth itself becomes more fluid. It's not unreasonable to think of a retaining wall as something that resists a hydraulic force.
Here is an interesting example of a concrete slab retaining wall. Clearly the concrete has been colored, either with paint or more likely with integral coloring agents. The designer was aware that a drain may help reduce some of the hydraulic force during a heavy rainstorm. They had moderately good success at including a cast stanchion to provide a base for a lamp. If you look closely, you can see the horisontal line that shows where the casting forms abutted. I suspect this wall is of fairly recent vintage, for the simple reason that the concrete texture is smooth, unlike most vintage Seattle concrete slab walls.
The following photograph is not actually a retaining wall, as there is no earth behind it. Perhaps there was once, which was the cause of the large vertical fissure. The reason I'm including this example is that you can see that ordinary boards, probably 1 X 6's were used as the forms. Just to the left of the crack is clearly seen a knot from the wood! It wasn't until after World War II that plywood became common in the building trades, so it's safe to assume that if you see a cast concrete wall like this, it was built before WWII.
